1. Field of the Invention
The present invention relates generally to methods for low temperature solvent absorption of acid gases from high pressure gas streams. More particularly, the present invention relates to a method for expanding a cleaned synthetic gas stream to provide at least a portion of the cooling requirement for the low temperature solvent absorption.
Synthetic gas streams produced by coal gasification are usually contaminated with sulfur, typically in the form of hydrogen sulfide, carbonyl sulfide, and the like. Before the synthetic gas can be used as a fuel for combustion or as a feed stock for chemical synthesis, it is usually necessary to remove such sulfur species to low levels.
Various processes are used for sulfur removal, including absorption processes which rely on contacting the synthetic gas stream with a physical or chemical solvent under conditions which promote absorption. Of particular interest to the present invention, physical solvents, such as lower monohydric alcohols, e.g. methanol, and polyhydric alcohols, e.g. ethylene glycol, may be contacted with the synthetic gas streams at low temperatures to effect the absorption of hydrogen sulfide. Such low temperature physical solvent absorption processes are commercialized under the tradenames Selexol (DMPEG), Ucarsol, Rectisol (low temperature), Purisol (NMP), and Fluor Solvent (propylene carbonate).
Low temperature solvent absorption of acid gas may be achieved in a conventional contactor, such as a tray or packed column, where the inlet solvent and/or the inlet synthetic gas are chilled and the column is insulated. Chilling of the solvent and/or inlet gas has usually been accomplished by direct or indirect heat exchange with a refrigerant which has been cooled in external refrigeration equipment. Although effective, the need to use external refrigeration equipment requires substantial energy and reduces the overall energy efficiency of the solvent absorption process.
It would therefore be desirable to provide a low temperature solvent acid gas absorption process having improved energy efficiency. In particular, it would be desirable to provide such low temperature acid gas absorption processes having a reduced requirement for external refrigeration. Even more particularly, it would be desirable to arrange the absorption process to employ energy available from the treated gas itself to provide for at least a portion of the necessary refrigeration requirement. Additionally, it would be desirable to arrange the absorption process to allow extraction of mechanical energy from the treated gas, where such mechanical energy can be used for producing electricity or other purposes.
2. Description of the Background Art
U.S. Pat. No. 4,019,314, discloses a coal gasifier which produces a synthetic gas which is fed to a scrubber-cooler. The scrubbed synthetic gas is fed to an expansion turbine, where mechanical energy is extracted. U.S. Pat. No. 4,524,581, describes a high pressure coal gasification process. U.S. Pat. No. 4,212,160 describes acid gas removal processes in combination with coal gasification. U.S. Pat. Nos. 4,2I1,540 and 4,155,988, describe the use of low temperature physical solvents for absorbing hydrogen sulfide from synthetic gases produced by coal gasification. None of the processes described in these patents disclose the expansion of gas with gas treating to reduce refrigeration requirements and increase efficiency.